Method of patenting steel wire



April 14, 1970 Filed Aug. s, 1968 H. GElPr-:L ETAL METHOD OF PATENTINGSTEEL WIRE 3 Sheets-Sheet 1 Hans Geipel Wilfried Heinemann INVENTORS.

Attorney April 14, 1970 H, GElPEL EVAL 3,506,468

METHOD 0F PATENTING STEEL WIRE 5 Sheets-Sheet 2 Filed Aug. 6, 1968INVENTORS t HANS GE//q-'L clau/:Rar Fonsme BY w/LFR/Eo HE/NEMANN TTRNFVApril 14, 1970 H. GEIPYEI. ETAL 3,506,468

METHOD 0F PATENTING STEEL WIRE Filed Aug. 6. 1968 3 Sheets-Sheet 5canonarq* :0A/mallen INVENTORS HANS GE/PEL, chE/Maar C/57242 BY W/LFQ/sa#lewe-MANN United States Patent O U.S. Cl. 14S-12.4 7 Claims ABSTRACT OFTHE DISCLOSURE Austenitic steel wire with a carbon content between about0.3 and 0.9% rolled at a temperature above the transformation point AC3,is cooled at a rate of at least 20 C. per second immediately aftercoming from the last stage of a hot-rolling mill, the first phase of thecooling past the GOS line being carried out by quenching while the finalcooling, down to a level between 480 and 580 C., takes place in auidized bed.

This application is a continuation-in-part of our copending applicationSer. No. 675,522 tiled Oct. 16, 1967.

Our present invention relates to a method of treating steel wire, toincrease its tensile, flexural and torsional strength.

In making wire of this type, e.g. as required for producing steel cablesor coil springs, the conventional practice is to wind the hot wire fromthe last rolling station into a coil, allowing the coil to cool andthereafter heat-treating the wire in a fused bath, such as molten lead.This type of heat treatment is generally referred to as patentingf aterm which may also be used more broadly for the cooling of wire in anymedium at a controlled rate from a level above the critical point AC3(transformation of ferrite to austenite) to a range in which austeniteis transformed into pearlite.

In order to satisfy the usual requirements of ductility, exibility andtensile as well as torsional strength, the wire so treated should have apredominantly sorbitic crystal structure. Sorbite is a fine-grainedvariant of pearlite and comes into existence upon transformation ofaustenitic steel at a temperature of approximately 550 C. If thetransformation occurs at a lower level, generally below 500 C., thepearlite crystals are still smaller and form a structure known asbainite. This structure is considerably harder than the sorbite andunsuitable for drawing. If, on the other hand, transformation is allowedto occur at temperatures above the level of substantially 550 C., thepearlite becomes progressively coarser as its crystals are surrounded bya ferrite skeleton; such a wire, typically obtained by patenting in air,has good ductility and torsional strength but does not withstand tlexureas well as does wire transformed in a range of about 500 to 550 C.

In our above-identified copending application we have disclosed a methodof patenting such a wire in a cooling medium of the lluidized-base type,i.e. a stream of carrier gas with entrained solid particles such asceramic granules of elevated heat-transfer coefficient (preferablybetween about 500 and 1000 cal./cm.2hr. C.). The particles may consist,for example, of magnesia and may range between 0.03 and 0.15 mm. indiameter, with a bulk weight of 1.5 to 5 g./cm.2. Hydrogen, carbonmonoxide or other relatively inert gases conventionally used inmetallurgical processes may serve as 3,506,468 Patented Apr. 14, 1970the carrier fluid. Though the temperature of the cooling medium (solidparticles and carrier gas) may be well below the bainite-formation levelof about 500 C. transformation is completed above that level because thewire is led out of the lluidized bed in a state of incipienttransformation before its temperature fall below the 500 C mark. Thismethod can be applied directly to wire coming hot from a rolling milland thus represents a more economical :process for obtaining the desiredsorbitic structure with substantial exclusion of bainite.

Our present invention, to the extent that is goes beyond the disclosureof our prior application Ser. No. 675,522, represents a furtherdevelopment of that method particularly designed for austenitic steelWire with a carbon content between about 0.3 and 0.9% (by weight). Itsobject is to provide a method of treating such wire in a mannerresulting, independently of the type of cooling medium employed, in astructure having the desired ductility and strength for the purposesspecied above.

This object is realized, in accordance with our present invention, byimmediately cooling the hot-rolled wire at a rapid rate of at least 20C. per second to a temperature within the austenite/pearlitetransformation range, i.e. a temperature lying generally between 500 and550 C. although its lower and upper boundaries may be around 480 and 580C., respectively. The forced-cooling process, which should start notlater than about one second after the wire has left the last rollingstage, should lower the temperature of the wire to a level below the GOSline of the iron-carbon equilibrium diagram within a few seconds andshould be terminated after not more than about l0 seconds from the timeof its inception.

According to a more specific aspect of our invention, the initialcooling phase (past the GOS line) is carried out by quenching in Waterwhile the subsequent cooling is performed in a fluidized bed asdescribed above.

A wire so treated has surprisingly high stress resistance along with thenecessary ductility allowing it to be drawn to the desired finaldiameter. Without wishing to commit ourselves to any definite theoriesin explaining these phenomena, we ascribe them to a freezing of themolecular structure produced by rolling which may be characterized by ahigh density of dislocations.

We have found that the treatment of wire by our present method resultsin a sorbitic structure comparable to that realizable, albeit atsubstantially lower production rates, with a bath of molten lead.Moreover, the treatment according to our invention is faster thanpatenting in air and tends to suppress the formation of the ferriteskeleton usually associated with air cooling.

After the wire has emerged from the lluidized bed, transformationproceeds to completion under substantially isothermic conditions, i.e.without the use of a cooling medium other than the surroundingatmosphere. To retard the cooling at this stage it is, however,desirable to shield the emerging wire by sheet-metal plates or the likereilecting its thermal radiation. The final cooling, subsequent totransformation, may also take place 1n air.

In order to stabilize the temperature of the emerging wire within thedesired range of approximately 500 .to 550 C., we prefer to measure thattemperature and to compare it with a predetermined value to compensatefor deviations therefrom by a corrective adjustment of the bedtemperature and/ or of the residence time of the wire in the uidizedbed. To control the temperature of the cooling medium, we prefer toremove particles continuously from the bed and to let them pass througha cooling chamber before returning them to the bed; this recirculationof the particles is best accomplished with the aid of a ow of carriergas which may itself be recirculated.

A plant suitable for carrying out the aforedescribed method comprises aconveyor, preferably in the form of an apertured belt, passing through achannel together with the stream of carrier gas and entrained solidparticles; the discharge end of the channel is provided with a gatethrough which the cooled wire may emerge while the particles areretained and form a nearly stationary accumulation around the exitingwire. The hot incoming wire may be deposited on the conveyor in asuccession of loops, advantageously with the aid of a transverselyoscillating dispenser as disclosed and claimed in our commonly ownedapplication Ser. No. 675,405, filed Oct. 16, 1967.

The invention will now be described in greater detail with reference tothe accompanying drawing in which:

FIG. 1 is a transformation diagram showing the conversion of austeniticsteel to sorbite by conventional means and by the process of ourinvention;

FIG. 2 is a somewhat diagrammatic Side-elevational view of a plant forcarrying out the process;

FIG. 3 is a fragmentary view similar to FIG. 2, showing a modification;and

FIG. 4 represents part of the iron-carbon-equilibrium diagram, includingthe GOS line.

In FIG. 1 we have shown at A and B the boundaries of theaustenite/pearlite transformation range for a typical steel wire of 5.5mm. diameter, made from unalloyed steel with a carbon content of 0.5%.Graph e represents an idealized process whereby the wire is rapidlycooled, from a starting temperature of 860 C., to a level of 550 C.which it reaches after 11/2 seconds and where the graph intersects theboundary curve A of the transformation range. After a further intervalof about 181/2 seconds, with gradual cooling to a point at or above 500C., the transformation to sorbite would be completed without theformation of appreciable quantities of bainite. Such an idealizedcooling process, e.g. with quenching in water, would be difficult torealize because of the problems of temperature control and appears to beimpractical for any but the thinnest wires.

It is widely assumed, even if not established by ncontrovertible proof,that the qualities of steel wire especially in regard to exure areimproved by an approximation of the conditions represented by graph e.This may be accomplished, to a certain extent, by the use of a bath ofmolten lead (graph a) which, in order to avoid the formation of bainite,should be maintained at a ternperature of about 500 C. so that the curveapproaches this level asymptotically; this type of treatment, completedafter 20` seconds, does not lend itself to the processing of hot wirecombining at relatively high speed from a rolling mill. Conventional aircooling (graph d) takes even longer and leads to incipienttransformation at a temperature close to 700 C., with resultingformation of a large-grain ferrite structure in the pearlite.

The treatment of wire in accordance with our present invention isrepresented by graphs b and c. Graph b illustrates the cooling byceramic granules of the aforedescribed type having a heat-transfercoefficient a=600 cal./m.2hr. C., as compared with a value a=1180 forthe lead bath of graph a. Graph c applies to ceramic particles witha=850. The particle temperature is maintained well below 5 00 C., yetcontact between the particle stream and the wire is terminated at apoint p or q, thus after 8 or 6 seconds, respectively, when the wiretemperature drops to a level of 520 C. The treatment then continuessubstantially isothermally for a further period of approximately 20seconds, to a point r well beyond the intersection of graphs b and cwith curve B, whereupon nal cooling proceeds in the open air (withoutany thermal shielding) as indicated by the joint portions b, c of thetwo graphs.

From the diagram of FIG. 4 it will be noted that the boundary betweenaustenite and the ferrite/ austenite mixture, represented by the lineGOS, lies at a level of approximately 750 C. for steel having a carboncontent of about 0.6%. In FIG. 1 the GOS line has been indicated at thatlevel and is shown to intersect the curves b and c at points where therate of cooling, as represented by the slopes of these curves, is wellover 20 C. per second.

Reference will now be made to FIG. 2 for a description of a plant inwhich the process described in connection with FIG. 1 can be performed.The plant comprises a uidized bed 1 confined within a tunnel 24, formingan elongated ow channel, to the vicinity of the upper run of an endlessconveyor belt 2 which is continuously driven by a motor 15 so that a hotwire 3, deposited thereon after leaving the last stage of a hot-rollingmill and preferably after preliminary quenching as indicated in FIG. 1,is transported on a downwardly sloping path from right to left. Wire 3passes through a guide tube 4 and a continuously rotating dispenser arm25, driven by a motor 26, whose rotation forms the wire into asuccession of loops deposited on the conveyor 2; the dispenser arm 25may be subject to continuous transverse oscillations at a frequencyrelated to the loop-deposition rate, as described in our copendingapplication Ser. No. 675,- 405, for the purpose of insuring optimumdistribution of the loops over the available conveyor surface. Belt 2,designed as a wire screen or other apertured member, transports theloops through a gate 8 at the discharge end of the channel, this gatebeing here shown as a simple shutter having a slot for the passage ofthe wire loops; a more elaborate gate, designed to prevent the loss ofsolid particles through the exit slot, has been disclosed in ourcommonly owned application Ser. No. 675,426 filed Oct. 16, 1967. Aperforated base 27 within tunnel 24 forms the lower boundary of bed 1and is connected to outlets of a manifold 10 through which a carriergas, as indicated by the arrows, is passed at longitudinally spacedlocations by way of the interstices of belt 2 into the space thereabove.The branch conduits of manifold 10 contain respective valves 9 forcontrolling the amount of gas thus introduced. A further valve 28controls the input from a compressor or other high-pressure source, notshown, whereas two other valves 29, 30 determine the proportion in whicha portion of the gas is branched off into a conduit 5 into which opensan outlet of a cooling chamber 6, the latter containing a coil 22transversed by a coolant. Conduit 5 opens into the tunnel 24 in thevicinity of the housing 23 of the dispenser arm 25.

Solid particles entrained by the gas stream accumulate in a pile justahead of the shutter 8 where the tunnel 24 is formed with a dischargeport 7 for these particles. A similar accumulation is formed at theentrance end of the tunnel by means of a stationary plate 31 underlyingthe upper run of conveyor belt 2 lbeneath an inlet branch 32 of conduit5. Port 7 communicates with a further conduit 33 which leads to the topof cooling chamber 6 and which may include means, such as a pump 34, topromote the return of solid particles from the discharge end of tunnel24 to the cooler. Another conduit 16, provided with a control valve 35,serves as a suction line to exhaust particles from the vicinity ofshutter 8 to a separator 18 whence they are returned to cooler 6 via apipe 21; the spent carrier gas drawn off by line 16, and by a branch 36thereof extending from the entrance end of the channel, is removed by apump 17 into a conduit 19 whence it may be discharged by way of a valve37 to the atmosphere or to the low-pressure side of the compressordelivering fresh gas to valve 28. A bypass 20, controlled by a valve 38,enables the recirculation of some 0r all of the gas to manifold 10.

In accordance with an important feature of our invention, a temperaturefeeler 11 just beyond shutter 8 senses the temperature of the emergingwire loops and feeds this information to a comparator 13 receiving areference signal from a storage device 12 adjusted to the desired exittemperature (e.g. 520 C.). Comparator 13 sets a controller 14 which, ifnecessary, adjusts the speed of motor to vary the residence time of thewire in the lluidized bed 1 in a manner compensating for any deviationsof its exit temperature from the preset reference value.

Dispensing arm 25 is, of course, representative of any convenient typeof loop depositor including, for example, devices of the type shown inU.S. Patents Nos. 3,056,433 and Re. 26,052.

The wire 3 exiting from gate 8, thermally shielded against excessiveradiant-heat losses by a tube 39 forming an extension of tunnel 24,continues on conveyor 2 in the ambient atmosphere until itstransformation has been completed (point r in FIG. l). Thereafter, itmay be aircooled more rapidly outside the tube 39, by the same oranother conveyor or without any conveyor at all, to room temperature.

In FIG. 3, where elements corresponding to those of FIG. 2 have beendesignated by the same reference numerals with addition of a prime mark,we have shown the temperature sensor 11 disposed ahead of shutter 8'.Sensor 11' ascertains the exit temperature of the wire in terms of thetemperature of the fluidized bed 1 at the discharge end of tunnel 24and, as before, communicates this information to a controller 14'; theoutput of this controller, in contradistinction to the previousembodiment, sets a servomotor 40 which adjusts a valve 41 to regulatethe amount of cooling lluid passing through coil 22 of chamber 6'. Thesystem operates otherwise in the same manner as the arrangement of FIG.2. Naturally, the control systems 11, 11 shown in FIGS. 2 and 3 couldalso be combined in a single plant.

EXAMPLE I Steel wire containing 0.58% C, 0.38% Mn, 0.24% Si, 0.01% P and0.02% S (all percentages by weight) is rolled to a diameter of 5.5 mm.at a temperature of y800" C. One second after leaving the last rollingstage, forced cooling of the wire is started, proceeding at an averagerate of 40 C. per second to a level of 520 C. and, this temperaturebeing maintained for 12 seconds while the transformation from gamma toalpha iron proceeds to completion. After pickling and rustproong('bonderizirlg), the wire is drawn without further heat treatment to adiameter of 1.8 mm., this corresponding to a deformation of about 90%.

Wire so drawn exhibited a tensile strength of 180 kg./mm.2 and`withstood 22 consecutive cycles of ilexing and straightening. A cableformed from six strands of seven such wires each was found to have aservice life two to four times as long as identical cables made fromconventionally lead-patented wire.

EXAMPLE II The procedure of Example I is followed, using a wire with acontent of 0.65% C, 0.55% Mn, 0.24% Si, 0.012%

P and 0.02% S, rolled to the same diameter of 5.5 mm. After pickling andbonderizing, the wire is drawn without further heat` treatment to adiameter of 2.2 mm., this corresponding to a deformation of about 85%.

Wire so drawn exhibited a tensile strength of 200 kg./mm.2, a torsionalstrength in terms of 40 consecutive cycles of reversing twist and a 50%constriction on rupture. Coil springs of 85 mm. diameter, 110 mm. lengthand four turns, formed from this wire, showed an axial compression ofonly 7% after 10,000 alternate cycles of compression and relaxation,compared with a for shortening in the case of identical springs ofconventionally lead-patented wire. An appreciable loss of stabilityoccurred only after 60,000 compression cycles, as compared withapproximately 40,000 cycles for the conventional spring.

We claim:

1. A method of treating austenitic steel wire with a carbon contentbetween substantially 0.3 and 0.9%, rolled at a temperature above thetransformation point AC3 which comprises the step of subjecting saidwire, immediately after rolling, to forced cooling at a rate of at least20 C. per second, the forced cooling being terminated in a temperaturerange between substantially 480 and 590 C.

2. A method as defined in claim 1 wherein the wire is quenched withwater in an initial phasev of the forcedcooling step, said initial phaseincluding a traverse of the GOS line of the iron-carbon-equilibriumdiagram.

3. A method as defined in claim 1 wherein the forced cooling is carriedout in a uidized bed at least from a level below the GOS line of theiron-caribon-equilibrium diagram.

4. A method as defined in claim 1 wherein the forcedcooling step startswithin one second after rolling and terminates within substantially 10seconds thereafter.

5. A method as dened in claim 1 wherein the wire, after cooling, issubjected to drawing without further heat treatment.

6. A method as defined in claim 5 wherein a multiplicity of wires sodrawn are twisted into a cable.

7. A method as defined in claim 5 wherein the wire so drawn is coiledinto a spring.

References Cited UNITED STATES PATENTS 2,019,445 10/1935 Crapo 148--1532,224,998 12/ 1940 Wood et al 148-153 3,231,432 1/1966 MCClean et al.148--153 L. DEWAYNE RUTLEDGE, Primary Examiner W. W. STALLARD, AssistantExaminer U.S. Cl. X.R.

